Natural gas is bought and sold based on its heating value. It is the BTU content that determines the monetary value of a given volume of natural gas. This BTU value is generally expressed in decatherms (one million BTU). In the determination of total heat value of a given volume of gas, a sample of the gas is analyzed and from the composition its heat value per unit volume is calculated. This value is generally expressed in BTU/cu ft. The typical range of transmission quality gas ranges between 1000 and 1100 BTU/cu ft. Production gas, storage facility gas, NGL, and new found Shale Gas can have much higher heating values up to or even exceeding 1500 BTU/cu ft.
There has been a long standing controversy between gas producers and gas transporters regarding entrained liquid typically present in most high BTU/cu ft. gas (rich or “wet” gas). Transporter tariffs require essentially liquid-free gas. Hydrocarbon liquid in the gas being transported causes operational and safety problems. The practice is to separate the liquid before entering a transport (pipe) line.
The API 14.1 standards (Manual of Petroleum Measurement Standards, 2006) scope does not include supercritical fluid (dense phase) or “wet gas” “(a term referenced by the Natural Gas industry as a gas that is at or below its hydrocarbon dew point temperature and/or contains entrained liquid), nor does the GPA 2166 standard (Obtaining Natural Gas Samples for Analysis by Gas Chromatography, 2005). In summary, there is no known standard which defines how to obtain a “representative sample” of a natural gas supply having entrained “liquid” hydrocarbon in any form.
Therefore to fully comply with the current industry standards, there is a need to prevent entrained liquids from entering sample systems. Membrane-tipped probes such as the A+ Corporation Genie Probe (see U.S. Pat. No. 357,304, U.S. Pat. No. 6,701,794, U.S. Pat. No. 6,904,816, U.S. Pat. No. 7,004,041, and U.S. Pat. No. 7,134,318) have been used for many years to shed entrained liquids inside pressurized pipelines. However, they do not have any physical block to stop entrained liquids, should the differential pressure be exceeded, forcing liquids through the coalescing elements, and into the sample system. The differential pressure needed to force liquids through the coalescing elements is a function of the surface tension of the liquid and the construction of the coalescing element. Sometimes man-made liquid chemicals are injected into the process like corrosion inhibitors, amine and carbon dioxide inhibitors as well as chemicals meant to dry the gas like alcohols and glycols. These liquid chemicals may have low surface tensions that could get past some coalescing elements. The liquid chemicals may combine with the sample to lower the surface tension of the sample making it easier for the sample to get past some coalescing elements. Also some coalescing elements may have temperature limitations. Therefore, there is a need for a physical block to stop these liquids from getting into the sample system.
Viewable (transparent) level indicators with floats/valves have been used for many different purposes through the years. For example, McKillop's level indicator with float valve has been used for concrete mixer water tanks (See U.S. Pat. No. 2,725,071) since the 1950s.
Another company, Welker Engineering, utilizes a device similar to the McKillop viewable (transparent) indicator. Welker has a viewable (transparent) liquid protection device (see U.S. Pat. No. 5,579,803) outside a pressurized pipeline to protect gas chromatographs during on-line sampling since 1995. However, these type devices cannot stop aerosols, or mist of liquid. They can only stop large slugs of liquids. The operator is encouraged to view the device to look for mist or aerosols that could get past the device. Then a separate device with a bypass or drain is needed to coalesce the mist or aerosol, and drain it to remove it. A bypass stream or drain is not desirable, since valuable process sample is vented to the atmosphere and/or drained on the ground. In addition, EPA regulations may prohibit such venting and draining of process fluids. Even if the vented gas and drained liquid is sent to a flare, the result must be monitored for safe emissions.
Further, viewable transparent devices such as McKillop's and Welker's are limited in pressure rating due to the transparent material of construction.
More recently, A+ Corporation introduced a liquid valve feature in a Genie Membrane Separator (see U.S. Pat. No. 7,555,964) where a valve is used in conjunction with a phase separation membrane/diaphragm. This method is not designed for conventional probe insertion into pressurized pipelines because the diaphragm diameter used to actuate the valve is large compared to the available opening in the pipeline for probe insertion. The small diameter diaphragms required to fit into the available hole in the pipeline are so small that the analytical flow rate is too limited to be of any practical use. Further, the device is neither transparent nor is the liquid viewable. Nonetheless, this device works well in high pressure applications outside the pipeline, before an analyzer, to protect gas analyzers during on-line sampling and spot sampling, when the safe maximum allowable differential pressure is exceeded so as to force liquids through the phase separating coalescing elements. The diaphragm has no size limitation outside the pipeline since there is no insertion requirement, and therefore no size constraint.
The Genie Membrane Separator device requires a bypass or drain to coalesce any mist or aerosol as a drain for removal. Again, a bypass stream or drain is not desirable since valuable process sample is vented to the atmosphere and/or drained on the ground. In addition, EPA regulations may prohibit such venting and draining of process fluids. Even if the vented gas and drained liquid is sent to a flare, the result must be monitored for safe emissions.
Further, any time liquid is removed from the source and transported into the sample system, the liquid distorts the true composition of the sample. It would be preferable to stop the liquid from ever entering the sample system, to prevent sample distortion and contamination which equates to wrong analysis, and very costly, incorrect monetary exchanges at custody transfer points, for both producers and transporters. In addition, it would also be desirable to have a device that does not need a bypass stream that must be vented to the atmosphere or need a drain that drains the liquids blocked onto the ground.